#!/usr/bin/env python3

# http://inamidst.com/saxo/
# Created by Sean B. Palmer

import itertools
import os
import struct
import sys
import time

def parse(name):
    # Specification from http://69.36.11.139/tzdb/tzfile-format.html
    # tzfile(5) also gives the information, though less clearly

    with open(name, "rb") as f:
        def get(struct_format):
            struct_format = "> " + struct_format
            file_bytes = f.read(struct.calcsize(struct_format))
            return struct.unpack(struct_format, file_bytes)
    
        header, version, future_use = get("4s c 15s")
    
        counts = {}
        for name in ("ttisgmt", "ttisstd", "leap", "time", "type", "char"):
            counts[name] = get("l")[0]
    
        transitions = get("%sl" % counts["time"])
        indices = get("%sB" % counts["time"])
    
        ttinfo = []
        for current in range(counts["type"]):
            ttinfo_struct = get("l?B")
            ttinfo.append(ttinfo_struct)
    
        abbreviations = get("%sc" % counts["char"])

    index = 0
    abbreviation_indices = {}
    for abbreviation in b"".join(abbreviations).split(b"\x00"):
        abbreviation_indices[index] = abbreviation.decode("us-ascii")
        index += len(abbreviation) + 1

    for current, ttinfo_struct in enumerate(ttinfo):
        replacement = abbreviation_indices[ttinfo_struct[2]]
        ttinfo[current] = (ttinfo_struct[0], ttinfo_struct[1], replacement)

    offset, dst, abbreviation = ttinfo[0]
    tzinfo = [(None, offset, dst, abbreviation)]
    for transition, index in zip(transitions, indices):
        offset, dst, abbreviation = ttinfo[index]
        tzinfo.append((transition, offset, dst, abbreviation))

    return tzinfo

def window(seq, n=2):
    "Returns a sliding window (of width n) over data from the iterable"
    "   s -> (s0,s1,...s[n-1]), (s1,s2,...,sn), ...                   "
    it = iter(seq)
    result = tuple(itertools.islice(it, n))
    if len(result) == n:
        yield result    
    for elem in it:
        result = result[1:] + (elem,)
        yield result

def main(zone):
    import re
    regex_zone = re.compile("(?i)^[A-Z0-9_]+/[A-Z0-9_]+$")
    if regex_zone.match(zone):
        now = time.time()
        name = "/usr/share/zoneinfo/" + zone
        if not os.path.isfile(name):
            print("Unknown zone name")
        tzinfo = parse(name)
        for a, b in window(tzinfo):
            if now < (b[0] or 0):
                offset = a[1] / 3600
                if not (offset % 1):
                    offset = int(offset)
                print("%+d (%s)" % (offset, a[3]))
                return
    else:
        print("Not a valid zone name")

if __name__ == "__main__":
    main(sys.argv[1])